1. Field of the Invention
The present invention relates to a fixing device and an image forming apparatus incorporating the same, and more particularly, to a fixing device that fixes a toner image in place on a recording medium with heat and pressure, and an electrophotographic image forming apparatus, such as a photocopier, facsimile machine, printer, plotter, or multifunctional machine incorporating several of those imaging functions, incorporating such a fixing device.
2. Discussion of the Background
In electrophotographic image forming apparatus, such as photocopiers, facsimiles, printers, plotters, or multifunctional machines incorporating several of those imaging functions, an image is formed by attracting toner particles to a photoconductive surface for subsequent transfer to a recording medium such as a sheet of paper. After transfer, the imaging process is followed by a fixing process using a fixing device, which permanently fixes the toner image in place on the recording medium by melting and settling the toner with heat and pressure.
Various types of fixing devices are known in the art, most of which employ a pair of generally cylindrical, looped belts or rollers, one being heated for fusing toner (“fuser member”) and the other being pressed against the heated one (“pressure member”), which together form a heated area of contact called a fixing nip through which a recording medium is passed to fix a toner image onto the medium under heat and pressure.
One conventional type of fuser assembly employed in the fixing device is an endless belt looped for rotation around a generally cylindrical, stationary heat pipe typically formed of a thin sheet of thermally conductive metal, with a small gap or clearance on the order of approximately 1 mm or less left between the adjoining surfaces of the belt and the heat pipe. The heat pipe has a heater inside to conduct or carry heat over its circumference, from which heat is radially transferred to the length of the fuser belt rotating around the heat pipe.
Using a thin-walled conductive heat pipe allows for heating the fuser belt swiftly and uniformly, resulting in shorter warm-up time and first-print time required to complete an initial print job upon startup, and high immunity against printing failures caused by insufficient heating of the fixing nip in high-speed application. For obtaining an extreme thinness, the heat pipe is typically formed by bending a sheet of thermally conductive material or metal through metalworking process into the require shape.
To date, a generally cylindrical, open-sided heat pipe is available for use in the conventional fixing device, which comprises a thin sheet of metal formed into a rolled configuration with two opposed longitudinal edges thereof spaced apart from each other to obtain a substantially C-shaped cross-section with an elongated opening on one side thereof. The open-sided heat pipe is used in combination with a separate fuser pad held stationary in its side opening outside the pipe interior and inside the loop of a fuser belt entrained around the heat pipe. When assembled, the open-sided heat pipe has its open side facing a pressure member extending parallel to the length of the pipe, so that the fuser pad is pressed against the pressure member through the thickness of the fuser belt to form a fixing nip.
Provision of the separate fuser pad enables the open-sided heat pipe to operate substantially in isolation from the pressure member, which can thus maintain its generally cylindrical configuration without bending or bowing away from the fixing nip under nip pressure, even where the heat pipe is extremely thin-walled to obtain high thermal efficiency in heating the fuser belt. Such stability against deformation of the heat pipe maintains a proper clearance between the fuser belt and the heat pipe, which in turn protects the fuser belt against damage and failure and results in proper operation of the fixing device.
One drawback encountered when using an open-sided heat pipe is the difficulty in establishing an adequate, uniform clearance between the fuser belt and the heat pipe during manufacture. The problem arises primarily due to dimensional variations in the open-sided heat pipe, whose generally cylindrical shape is difficult to fabricate with precision and vulnerable to deformation during handling, leading to variations in the size of the gap between the fuser belt and the heat pipe. Process variations occurring during assembly of separate pieces of fuser equipment also contribute to variations in the belt-to-pipe gap.
Failure to provide a proper clearance between the fuser belt and the heat pipe results in defects and failures of the fixing device. For example, too small a belt-to-pipe gap causes the fuser belt to rub against the heat pipe locally and intensively, resulting in various problems, such as scuffing of or other damage to the fuser belt, grating noise caused during operation, or disturbed motion of the fuser belt sliding against the heat pipe. On the other hand, too large a belt-to-pipe gap hinders transfer of heat from the heat pipe to the fuser belt, and destabilizes rotation of the fuser belt around the heat pipe.
Another drawback associated with the open-sided heat pipe is the difficulty in handling the fuser assembly for repair or replacement. That is, providing the heat pipe, the fuser pad, and the fuser belt (which are in most cases consumable) as separate pieces of fuser equipment results in cumbersome disassembly and assembly of the fixing device where those consumables need maintenance or replacement.
Still another drawback is that the fuser pad tends to bow or bend in the axial direction under pressure from the pressure roller. Such tendency to deformation is inherent in a typical design where the fuser pad has a pair of axial, longitudinal ends thereof secured to a frame of the fixing device without provision of other fastening structure that holds an axial center of the fuser pad in place due to the presence of the fuser belt entrained around the heat pipe. Deformation of the fuser pad results in variations in the width and strength of the fixing nip in the axial direction, leading to poor imaging performance of the fixing device.
To overcome the drawbacks of the conventional configuration, another type of heat pipe has been proposed that comprises a completely closed cylindrical pipe formed by bending a sheet of metal into a rolled configuration with two opposed longitudinal edges thereof bent inward and joined together to obtain a completely closed cross-section with an elongated side slot on one side thereof.
The completely closed heat pipe has a separate fuser pad held stationary in its side slot outside the pipe interior and inside the loop of a fuser belt entrained around the heat pipe, as well as a reinforcing member held stationary against the inner circumference of the pipe for reinforcement purposes. When assembled, the completely closed heat pipe has its outer circumference facing a pressure member extending parallel to the length of the pipe, with the reinforcing member supporting those portions of the pipe circumference pressed against the pressure member to form a fixing nip.
Compared to the open-sided configuration, the completely closed heat pipe is exempt from dimensional variations, as the opposed edges of the bent sheet of material are joined together to retain the cylindrical shape during handling. Moreover, accommodating the fuser pad in the side slot of the heat pipe allows for integrating the heat pipe and the fuser pad into a single integrated unit. Such integration of fuser assembly prevents variations in the gap between the heat pipe and the fuser belt, while facilitating assembly and disassembly of the fuser belt, the fuser pad, and the heat pipe during repair or replacement of those consumables. Furthermore, providing the fuser pad with the reinforcing member inside the heat pipe prevents deformation of the fuser pad and the heat pipe under pressure from the pressure roller.
Although generally successful for its intended purpose, the completely closed heat pipe is still susceptible to deformation when adapted to fast printing process. That is, designing a fixing device for high-speed operation involves reducing the overall size of the fuser belt and the heat pipe and thinning the wall of heat pipe to minimize warm-up time, as well as increasing nip pressure to accelerate fusing of toner. Such optimization for high-speed application, however, makes the heat pipe as well as the fuser belt and the reinforcing member more vulnerable to deformation than those used in relatively slow imaging processes.
As mentioned above, deformation of the heat pipe causes variations in the size of the gap between the fuser belt and the heat pipe, which result in various defects and failures of the fixing device. The problem becomes particularly noticeable where the fixing device incorporates the capability to adjust the length and pressure of the fixing nip by moving the pressure member relative to the heat pipe, which can stress the heat pipe upon touching or striking the pressure member at different pressures.